Process for producing oxygen partial pressure detecting part of resistance oxygen sensor

ABSTRACT

The present invention provides a method of manufacturing a porous thick film of an oxide that has extremely few cracks and can be satisfactorily used as an oxygen partial pressure detecting part of an oxygen sensor. The present invention relates to a method of manufacturing such a porous thick film as an oxygen partial pressure detecting part of a resistive oxygen sensor comprising taking a fine particle powder of an oxide containing cerium oxide as a raw material powder, preparing a paste containing the oxide, printing the paste onto a substrate by screen printing, calcining and sintering, the method comprising a step of carrying out heat treatment to effect particle growth from the average particle diameter of the raw material powder to a particle diameter less than the average particle diameter of the ultimately obtained thick film, a step of mixing the particle growth-effected powder with a solvent, a step of dispersing agglomerated particles in the solvent, a step of removing a precipitate, a step of evaporating off the solvent, and a step of mixing the resulting oxide with an organic binder to obtain the paste.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of manufacturing a porousthick film to be used in an oxygen partial pressure detecting part of aresistive oxygen sensor, and more specifically relates to a ceriumoxide-based porous thick film as an oxygen partial pressure detectingpart of an oxygen sensor that measures oxygen partial pressure and isused in an air-fuel ratio feedback control system for controlling theair-fuel ratio in the exhaust gas of an automobile or the like, so as toimprove the exhaust gas purification rate and the fuel efficiency, and amethod of manufacturing the cerium oxide-based porous thick film. Here,the air-fuel ratio is the ratio of air to fuel, there being a one-to-onerelationship between the oxygen partial pressure and the air-fuel ratio.

2. Description of the Related Art

Hitherto, as oxygen gas sensors for automobiles and the like, solidelectrolyte ones have predominantly been used (see, for example,Japanese Patent Application Laid-open No. S55-137334/1980). With thistype of sensor, the difference in the oxygen partial pressure between areference electrode and a measuring electrode is measured as anelectromotive force, and hence a reference electrode is required; therehas thus been a problem that the structure is complex, and it isdifficult to make the sensor small. To overcome this problem, resistiveoxygen gas sensors that do not require a reference electrode have beendeveloped (see, for example, Japanese Patent Application Laid-open No.S62-174644/1987). Briefly explaining the measurement principle of such aresistive oxygen gas sensor, first when the oxygen partial pressure ofthe ambient environment changes, the oxygen vacancy concentration in anoxide semiconductor changes. There is a one-to-one relationship betweenthe resistivity or electrical conductivity of the oxide semiconductorand the oxygen vacancy concentration, the resistivity of the oxidesemiconductor changing as the oxygen vacancy concentration changes. Bymeasuring the resistivity, the oxygen partial pressure of the ambientenvironment can thus be determined.

However, with such a resistive oxygen gas sensor, there has been aproblem that the responsiveness of the output upon the oxygen partialpressure changing is poor (see, for example, Japanese Patent ApplicationLaid-open No. H07-63719/1995). Moreover, titanium oxide has been used asthe oxide semiconductor of resistive oxygen gas sensors, but thismaterial has had the problem of being poor in terms of durability andstability. To overcome these problems, the present inventors havecarried out research and development into resistive oxygen gas sensorsthat use cerium oxide, the reason being that cerium oxide is durable incorrosive gases (see E. B. Varhegyietal., Sensors and Actuator B, 18-19(1994) 569). Through this research and development, the presentinventors have discovered that with a resistive oxygen sensor usingcerium oxide, the response speed can be improved by making the averageparticle diameter of the cerium oxide be as small as 200 nm and also beadding zirconium oxide to the cerium oxide, and have previously filed apatent application regarding this.

With the above inventions, a thick film is preferable as the form of theoxygen partial pressure detecting part, and to improve the responsespeed, the particle diameter of this thick film must be made low. Amethod of manufacturing such a thick film is as follows. A powdercomprising fine particles of an oxide is first taken as a raw material,a paste containing this raw material is prepared, this paste is appliedonto substrate by screen printing, and sintering is carried out, wherebya porous thick film as an oxygen partial pressure detecting part of aresistive oxygen sensor is manufactured. Here, the operating temperatureof the oxygen sensor will reach 1000° C. as a maximum. Thescreen-printed article must thus be sintered at a temperature exceeding1000° C. in advance so that particle growth will not occur attemperatures up to 1000° C.

With the above inventions, the raw material powder has been preparedusing a spray pyrolysis method. With this method, regarding theproperties of the fine particles, agglomeration is not prone tooccurring, and moreover the fine particles of the raw material powderhave a relatively high particle diameter (at least approximately 40 nm);it has thus been possible to obtain a porous thick film having extremelyfew cracks merely by preparing a paste using this raw material powderand screen printing. However, with the spray pyrolysis method, there hasbeen a problem that the amount of powder manufactured per unit time islow. On the other hand, fine particles of cerium oxide or an oxidehaving cerium oxide as a principal component thereof can be obtainedusing another method (see Japanese Patent Application Laid-open No.2002-255515/2002). This methodhas as the primary objective thereofobtaining extremely fine particles, and is a method enabling massproduction, and hence can be said to be better than the spray pyrolysismethod from an industrial standpoint. However, a powder obtained usingthis method is extremely fine, and moreover the fine particles areagglomerated; in the case of manufacturing a porous thick film from apaste in which such a powder and a vehicle (organic binder) are merelymixed together, there is thus a problem that there are many cracks, andhence the resistance of the thick film increases. It is undesirable forthe resistance to increase, since then the measuring circuitry formeasuring the resistance becomes complex. Moreover, there is a problemthat in the case that the cracks are very severe, there is no electricalconductivity, and hence use as an oxygen sensor is not possible, andmoreover there has also been a problem that the thick film readily peelsaway from the substrate because of the cracks.

SUMMARY OF THE INVENTION

In view of the prior art described above, it is an object of the presentinvention to thoroughly resolve the problems thereof, and provide aporous thick film of cerium oxide or an oxide having cerium oxide as aprincipal component thereof that has extremely few cracks and can besatisfactorily used as an oxygen partial pressure detecting part of anoxygen sensor, and a method of manufacturing such a porous thick film.Moreover, ideally, it is an object of the present invention to provide amethod of manufacturing a porous thick film having an average particlediameter of not more than 200 nm.

To attain the above objects, the present invention is constituted fromthe following technical means.

(1) A method of manufacturing a porous thick film as an oxygen partialpressure detecting part of a resistive oxygen sensor comprising taking afine particle powder of an oxide containing cerium oxide as a rawmaterial powder, preparing a paste containing the oxide, printing thepaste onto a substrate by screen printing, calcining and sintering, themethod comprising: a heat treatment step of carrying out heat treatmentto effect particle growth from the average particle diameter of the rawmaterial powder to a particle diameter less than the average particlediameter of the ultimately obtained thick film; a step of mixing theparticle growth-effected powder with a solvent; a step of dispersingagglomerated particles in the solvent; a step of removing a precipitate;a step of evaporating off the solvent; and a step of mixing theresulting oxide with an organic binder to obtain the paste.

(2) The method according to (1) above, wherein the average particlediameter of the porous thick film is not more than 200 nm.

(3) The method according to (1) above, wherein the average particlediameter of the particle growth-effected powder obtained through theheat treatment step is at least 45 nm.

(4) The method according to (1) above, wherein the average particlediameter of the raw material powder before the heat treatment step is atleast 10 nm but less than 45 nm.

(5) The method according to (1) above, wherein the raw material powderis subjected to heat treatment at 880° C. to 920° C. in the heattreatment step.

(6) The method according to (1) above, wherein the proportion by weightof the oxide in the paste is adjusted to 10 to 30 wt %.

(7) The method according to (1) above, wherein the fine particle powderof an oxide containing cerium oxide is a fine particle powder of anoxide containing cerium oxide and zirconium oxide.

(8) A cerium oxide-based porous thick film as an oxygen partial pressuredetecting part of a resistive oxygen sensor, the porous thick filmmanufactured using the method according to any of (1) through (7) above,whereby the porous thick film has few cracks, has an average particlediameter of not more than 200 nm, and has an electrical conductivity ofat least 10⁻³ S/m at 800° C.

Next, the present invention will be described in more detail.

A flowchart of a manufacturing method of the present invention is shownin FIG. 1. In the present invention, cerium oxide, or an oxide havingcerium oxide as a principal component thereof is used as a raw material.Specifically, an ‘oxide having cerium oxide as a principal componentthereof’ is, for example, an oxide having cerium oxide as a principalcomponent thereof and containing zirconium oxide, titanium oxide,germanium oxide, hafnium oxide or the like, more preferably an oxidehaving cerium oxide as a principal component thereof and containingzirconium oxide. In the case that the secondary component is moreabundant than the cerium oxide, the properties of the powder will begreatly different to those of cerium oxide, and hence it is preferablefor the secondary component concentration to be not more than 40 mol %.Moreover, the fine particles of the raw material preferably have anaverage particle diameter of 10 to 20 nm, with it being acceptable forthere to be a spread of particle diameters. The method of the presentinvention can be suitably applied to a powder of an oxide as describedabove comprising fine particles having a low particle diameter (lessthan 40 nm) that readily agglomerate. Examples of methods ofmanufacturing such oxide fine particles include, for example, aprecipitation method, a coprecipitation method, and a hydrothermalsynthesis method. With the precipitation method or coprecipitationmethod, a precipitate containing a hydroxide, water and so on is heatedin air, whereby an oxide powder can be obtained. In the case of ceriumoxide, the oxide can be obtained upon heating at a temperature of 600°C. The heat treatment step for changing the precipitate containing thehydroxide, water and so on into the oxide, and a heat treatment step foreffecting particle growth of the present invention can thus be carriedout consecutively.

With the present invention, first, to effect particle growth to aparticle diameter less than the average particle diameter of the thickfilm to be ultimately obtained, a raw material powder as described aboveis subjected to heat treatment in a heat treatment step; this is becauseif particle growth is not effected, then cracks will arise in the thickfilm after the sintering step described later (see Example 1 andComparative Example 2). Moreover, the reason for making the particlediameter to which the particle growth is effected be less than theaverage particle diameter of the thick film to be ultimately obtained isthat it is not possible to reduce the particle diameter in the finalsintering step. The temperature in the heat treatment step is preferablyat least 800° C., this being because at a lower temperature particlegrowth will not occur. Furthermore, in the case that the averageparticle diameter of the thick film ultimately obtained is made to be,for example, 100 nm, the temperature in the heat treatment step ispreferably 880 to 920° C. This is because with sintering atapproximately 950° C. or more, particle growth will occur to a particlediameter exceeding 100 nm (see Example 2), and hence it will beimpossible to make the average particle diameter of the thick filmultimately obtained be not more than 100 nm.

In general, the temperature in the heat treatment of the powder is setto be lower than the sintering temperature in the sintering carried outafter the screen printing. Consequently, in the case of making theaverage particle diameter of the thick film ultimately obtained be, forexample, 100 nm, it is sufficient if the particle diameter to which theparticle growth is effected in the heat treatment step is at least 45nm. As shown in Example 1 described later, in the case that the averageparticle diameter of the particle growth-effected powder is 48nm, aporous thick film having extremely few cracks is obtained. It is obviousthat even in the case of a particle diameter higher that this, a porousthick film having no cracks can be obtained, and hence if the particlediameter to which the particle growth is effected in the heat treatmentstep is at least 45 nm, then a porous thick film having extremely fewcracks can be obtained. Next, a solvent is added to the raw materialoxide; the solvent is preferably an organic solvent that has a lowviscosity and is easily evaporated such as ethanol or toluene. This isbecause a solvent that can be easily evaporated in the subsequent stepof reducing the amount of the solvent is preferable. Next, the oxide istreated in the solvent using an ultrasonic homogenizer or the like, thusdispersing agglomerated particles. A powder obtained by theprecipitation method or coprecipitation method is characterized in thatthe fine particles are agglomerated. If the powder is made into a pastewhile the particles are still agglomerated, then the thick filmultimately obtained will be extremely bumpy, and hence will not readilystick to an electrode or the like. Moreover, agglomerated particles area cause of cracks arising. It is thus necessary to disperse agglomeratedparticles. Moreover, even if the particles were not agglomerated beforethe heat treatment, the particles will agglomerate through the heattreatment in the heat treatment step described above, and hence thisdispersion step is always required.

Next, the solvent containing the oxide is left to stand as is, andafter, for example, approximately 30 to 40 minutes, precipitate isremoved. The precipitate comprises still agglomerated particles, andprecipitates out through its own weight, and can thus be separated fromthe dispersed particles. Next, the solvent is evaporated off whileheating and while stirring. After that, an organic binder is added; anexample of an organic binder is a vehicle comprising a mixture of ethylcellulose and terpineol, but there is no limitation to this. The organicbinder is a liquid having a prescribed viscosity, and by adding this apaste of a viscosity enabling screen printing is obtained. In thecurrent step, the wt % of the oxide is adjusted to a prescribed value,whereby an oxide-containing paste is obtained. The wt % of the oxide is,for example, preferably 10 to 30 wt %. This is because it is thoughtthat if the proportion of the oxide in the paste is too high, then themixing will be uneven. Moreover, if this proportion is too low, then theamount used of the binder will be high, which will be wasteful.

Next, the paste is printed onto a substrate by screen printing. Here, aninsulating material is used for the substrate. Preferable examplesinclude alumina, magnesia and quartz, but there is no limitationthereto. Next, calcination is carried out at 300 to 600° C., thusremoving the organic binder. As the calcination atmosphere, air, or anoxidizing atmosphere such as oxygen is preferable. This step can beomitted in the case that the rate of temperature increase in thesubsequent sintering step is set to be gradual. Finally, sintering iscarried out at 1000 to 1200° C., whereby an oxygen partial pressuredetecting part of a resistive oxygen sensor is obtained. The sinteringatmosphere may be any of air, an oxidizing atmosphere such as or oxygen,or a reducing atmosphere such as hydrogen or carbon monoxide. Throughthe sintering, necks are produced between the fine particles to form aporous body, which is electrically conductive. The average particlediameter preferably becomes 50 to 200 nm, more preferably 50 to 100 nm.The porous thick film obtained through the above manufacturing methodhas extremely few cracks, and can be satisfactorily used as an oxygenpartial pressure detecting part of an oxygen sensor; specifically, theelectrical conductivity is at least 10⁻³ S/m at 800° C., an excellentoxygen partial pressure dependence is exhibited, and the response speedis also excellent.

In the heat treatment step described earlier, the particle diameter ofthe powder is made to grow through the heat treatment; as describedearlier, if such particle growth is not effected, then particle growthwill occur in the subsequent sintering stage, and volume shrinkage willoccur accompanying this. At this time, stress must be relaxed, and it isthought that this is why cracks arise in the thick film. Alternatively,it is thought that this may be because the particle diameter is low, andhence the mixing with the organic binder becomes uneven, whereby placeswhere the fine particles are abundant and places where the fineparticles are sparse arise. In any case, to prevent such cracking, it isnecessary to effect particle growth in advance. Next, in the step ofdispersing agglomerated particles described earlier, particles that haveagglomerated in the solvent are dispersed; in the case that the particlediameter of the fine particles is low, even in the case of particleshaving no agglomeration at the raw material stage, agglomeration arisesupon effecting particle growth in the heat treatment step, and hencethis dispersion step is required.

Particles that are still agglomerated after this step can be removed asa precipitate in the step of removing the precipitate described earlier.Note that one can also envisage loosening the agglomeration through amechanical method such as pulverization, but cerium oxide is also usedas a polishing agent, and hence upon mechanical pulverization,impurities will be expected to get in; a mechanical method such aspulverization therefore cannot be used. In the step of preparing thepaste, if the wt % of the oxide is too high, then the proportion of theoxide out of the whole will be high, in which case the mixing betweenthe organic binder and the oxide will be uneven, and hence cracks and soon will become prone to occurring. Moreover, if the wt % is too low,then the proportion of the oxide in the paste will be low, and hence alarge amount of the paste will be required, which will be wasteful. Thewt % of the oxide is thus preferably 10 to 30 wt %.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flowchart of a method of the present invention;

FIG. 2 shows a scanning electron microscope (SEM) photomicrograph of araw material powder comprising cerium oxide fine particles obtainedthrough a precipitation method before a heat treatment step aspretreatment in Example 1;

FIG. 3 shows a low-magnification SEM photomicrograph of the raw materialpowder comprising cerium oxide fine particles obtained through theprecipitation method before the heat treatment step as pretreatment inExample 1;

FIG. 4 shows an SEM photomicrograph of the raw material powder after theheat treatment step as pretreatment in Example 1;

FIG. 5 shows an SEM photomicrograph of a thick film manufactured using apaste in which the oxide wt % is 50 wt % in Example 1;

FIG. 6 shows an SEM photomicrograph of a thick film manufactured using apaste in which the oxide wt % is 30 wt % in Example 1;

FIG. 7 shows an SEM photomicrograph of a thick film manufactured using apaste in which the oxide wt % is 20 wt % in Example 1;

FIG. 8 shows an SEM photomicrograph of a thick film manufactured using apaste in which the oxide wt % is 10 wt % in Example 1;

FIG. 9 shows a high-magnification SEM photomicrograph of the thick filmmanufactured using the paste in which the oxide wt % is 30 wt % inExample 1;

FIG. 10 shows an SEM photomicrograph of a thick film obtained bysintering at 1100° C. in Comparative Example 1;

FIG. 11 shows a high-magnification SEM photomicrograph of a thick filmmanufactured using a paste in which the oxide wt % is 30 wt % inComparative Example 2; and

FIG. 12 shows an SEM photomicrograph of a powder for the case that apowder comprising cerium oxide fine particles obtained using aprecipitation method was taken as a raw material and the pretreatmenttemperature was made to be 950° C. in Example 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Next, the present invention will be described concretely throughexamples; however, the present invention is not limited by the followingexamples whatsoever.

EXAMPLE 1

In the present example, a powder comprising cerium oxide fine particlesobtained through a precipitation method was used as a raw material.Briefly describing the method of preparing the powder using theprecipitation method, first a cerium nitrate aqueous solution wasprepared. Next, ammonia water was added, whereby a precipitate wasproduced. This precipitate was mixed with carbon, and heated for 4 hoursat 600° C. in air, thus obtaining the powder. Scanning electronmicroscope (SEM) photomicrographs of the powder (the raw material beforethe heat treatment step) are shown in FIGS. 2 and 3. The raw materialhad a particle diameter of 10 to 20 nm, and was agglomerated. Next, as apretreatment step, the powder was subjected to heat treatment for 4hours at 900° C. in air. An SEM photomicrograph of the raw materialafter this step is shown in FIG. 4. Particle growth occurred, theaverage particle diameter becoming 48 nm.

Next, ethanol was added as a solvent in a proportion of 50 ml per 1 g ofthe raw material. Dispersion was then carried out for 10 minutes usingan ultrasonic homogenizer. After the dispersion, the ethanol containingthe raw material was left to stand for 40 minutes, and then the stillagglomerated precipitate was removed. Ethanol was then again added sothat the volume of the ethanol containing the raw material became 50 ml,and then dispersion was again carried out for a further 10 minutes usingan ultrasonic homogenizer. After the dispersion, the ethanol containingthe raw material was left to stand for 30 minutes, and then theprecipitate was removed. At this stage, the weight of still agglomeratedprecipitate removed was 0.3 to 0.5 g per 1 g of the raw materialinitially used.

Next, the ethanol containing the oxide was heated to approximately 60°C. while stirring with a stirrer, thus evaporating the ethanol off.After that, a vehicle having a viscosity of approximately 3000 mPas wasadded as an organic binder. A mixture of ethyl cellulose and terpineolwas used as the vehicle. Here, the wt % of the oxide contained in thepaste was adjusted to a prescribed oxide wt %, that is 10, 20, 30 or 50wt %. In this way, pastes each comprising a mixture of the oxide and theorganic binder was obtained. Next, each paste was printed onto analumina substrate by screen printing. After the printing, drying wascarried out at 150° C. After that, screen printing was again carried outover the previously printed site, and then drying was again carried out.This was repeated a further two times, so that printing was carried outfour times in total.

After that, the printed article was calcined for 5 hours at 500° C. inair, and was then sintered for 2 hours at 1050° C. in air, thusobtaining a thick film as an oxygen partial pressure detecting part. SEMphotomicrographs of the thick films obtained in this way are shown inFIGS. 5 to 8. FIGS. 5, 6, 7 and 8 show respectively the SEMphotomicrographs of the thick films obtained using the pastes having anoxide wt % in the paste of 50, 30, 20 and 10 wt %. FIG. 9 shows anenlargement of FIG. 6. Moreover, as Comparative Example 1, the rawmaterial powder used in Example 1 was put into the vehicle (organicbinder) without being dispersed first, and mixing was carried out toobtain a paste, the paste was printed, calcination was carried out for 5hours at 500° C. in air, and then sintering was carried out for 2 hoursat 1100 or 1000° C. in air. FIG. 10 shows the thick film obtained bysintering at 1100° C.

Furthermore, as Comparative Example 2, thick films were manufacturedfrom the raw material powder used in Example 1 by not carrying out theheat treatment step but then carrying out the steps from the step ofmixing with ethanol onwards under the same conditions as in Example 1.Pastes having an oxide wt % of 10, 30 or 50 wt % were used. FIG. 11shows an SEM photomicrograph of the thick film with an oxide wt % in thepaste of 30wt %. For the thick films of Comparative Example 1,regardless of whether the sintering was carried out at a temperature of1100° C. or 1000° C., there were many cracks as shown in FIG. 10.Moreover, the surface was rough, and what appeared to be lumps of stillagglomerated particles were observed. It was thus found that with apaste manufactured merely be mixing as in Comparative Example 1, in thecase that sintering is carried out at 1000 to 1100° C., many cracksarise. Moreover, for the thick films of Comparative Example 2,regardless of the oxide wt %, many cracks arose as shown in FIG. 11.Moreover, the porous thick film readily peeled away from the substrate.This means that there would be no durability when using as a sensor.

On the other hand, with the thick films of Example 1, compared withComparative Examples 1 and 2, there were few cracks, and in particularthere were extremely few cracks with an oxide wt % of 10 to 30 wt %.Furthermore, at 20 wt %, a porous thick film with no cracks wasobtained. Moreover, as shown in FIG. 9, the thick film was extremelyporous. Moreover, the average particle diameter was 90 nm. A resistiveoxygen sensor having the porous thick film shown in FIGS. 6 and 9 as anoxygen partial pressure detecting part was manufactured. A platinumelectrode having a comb shape was formed on the porous thick film bysputtering, a platinum wire was attached thereto, and the resistance ofthe porous thick film was measured as the sensor output using atwo-terminal method. The oxygen sensor was placed in a sample chamber inan electric furnace in which the oxygen partial pressure could bechanged, and the resistance of the porous thick film, and the oxygenpartial pressure dependence thereof were investigated. Moreover, toinvestigate the response time, a rapid response evaluation apparatusenabling the oxygen partial pressure to be changed rapidly be changingthe total pressure rapidly was used. As sensor property evaluationresults, the resistance R and the electrical conductivity σ of theporous thick film at an oxygen partial pressure of 1.0 atm are shown inTable 1. In this way, it was found that a value of from a few tens of kΩto a few tens of MΩ was exhibited, and there was electricalconductivity. In the case that there were cracks as in ComparativeExample 2, there was no electrical conductivity, and use as a sensor wasnot possible. Moreover, even if there is slight electrical conductivity,because the electrical resistance is high, the circuitry for measuringthe resistance and the apparatus becomes complex, which is undesirable.A porous thick film having as few cracks as possible is thus preferableas an oxygen partial pressure detecting part. TABLE 1 Temperature (° C.)R (MΩ) σ(S/m) 600 19.7 5.0 × 10⁻⁴ 700 3.84 2.2 × 10−3 800 0.604 1.0 ×10−2 900 0.127 4.7 × 10−2 1000 0.0307 1.8 × 10−1

Next, the oxygen partial pressure dependence over an oxygen partialpressure range from 0.010 atm to 1.0 atm is shown in Table 2. n in Table2 is that in R ∝ p^(1/n), a small value thereof indicating that theoxygen partial pressure dependence is great. Here, P is the oxygenpartial pressure. At 600° C., n was 9.1, and hence the oxygen partialpressure dependence was somewhat low, but at the other temperatures, nwas from 5.5 to 6.5. Moreover, the response time (90%) at 800° C. was nomore than 20 ms, and hence the response speed was fast, satisfactorilyenabling use as an oxygen sensor. TABLE 2 Temperature (° C.) n(R∝P^(1/n)) 600 9.1 700 6.5 800 5.6 900 5.5 1000 5.8

EXAMPLE 2

A powder comprising cerium oxide fine particles obtained through aprecipitation method was used as a raw material, and the heat treatmenttemperature in the pretreatment was made to be 950° C.; an SEMphotomicrograph of the powder in this case is shown in FIG. 12. Particlegrowth has occurred to over 100 nm. It was thus found that in the caseof manufacturing a thick film for which the average particle diameter ofthe ultimately obtained thick film is to be 100 nm, a sinteringtemperature in the pretreatment step of 950° C. is too high.

EXAMPLE 3

In the present example, a powder was prepared using the followingprocedure. First, a cerium nitrate aqueous solution was prepared. Next,ammonia water was added, whereby a precipitate was produced. Thisprecipitate was mixed with carbon, and heated for 4 hours at 900° inair, thus obtaining the powder. In this heat treatment step, the heattreatment step for changing the precipitate containing the hydroxide,water and soon into the oxide and the heat treatment step for effectingparticle growth were carried out consecutively. The powder was thenmixed with ethanol, and then a thick film was manufactured using thesame method as in Example 1. The wt % of the oxide contained in thepaste was made to be 20 wt %. The porous thick film manufactured in thisway contained hardly any cracks, and hence it was found even if the heattreatment step for changing the precipitate containing the hydroxide,water and so on into the oxide and the heat treatment step for effectingparticle growth are carried out consecutively, the same effects areobtained.

EXAMPLE 4

A cerium nitrate aqueous solution and a zirconium oxynitrate aqueoussolution were mixed together in a Ce:Zr ratio of 8:2, thus obtaining amixed aqueous solution. Ammonia water was added to this mixed aqueoussolution, thus bringing about coprecipitatation. Next, the precipitatewas mixed with carbon, and heated for 4 hours at 900° C. in air, wherebyheat treatment for changing the precipitate containing a hydroxide,water and so on into an oxide, and the heat treatment step for effectingparticle growth were carried out consecutively. In this way, a powder ofcerium oxide containing 20mol % of zirconium oxide was obtained. Thiswas mixed with ethanol, and then thick films were manufactured using thesame method as in Example 1. The wt % of the oxide contained in thepaste was made to be 10 or 20 wt %. The cerium oxide porous thick filmscontaining 10 mol % of zirconium oxide manufactured in this way werechecked using SEM photomicrography, whereupon it was found that hardlyany cracks were contained; even at 20 mol %, it was found that therewere few cracks, and hence it was found that the present invention canalso be applied to cerium oxide containing zirconium oxide.

INDUSTRIAL APPLICABILITY

As described in detail above, the present invention relates to a methodof manufacturing a porous thick film as an oxygen partial pressuredetecting part of a resistive oxygen sensor. According to the presentinvention, the following remarkable effects are produced: 1) taking as araw material a powder comprising fine particles of cerium oxide or anoxide having cerium oxide as a principal component thereof manufacturedusing a precipitation method enabling mass production, there can bemanufactured a porous thick film of cerium oxide or an oxide havingcerium oxide as a principal component thereof that has extremely fewcracks, has an average particle diameter of not more than 200 nm, andcan be satisfactorily used as an oxygen partial pressure detecting partof an oxygen sensor; 2) with a conventional method, in the case of usingas a raw material a powder comprising fine particles that have a lowparticle diameter (less than 40 nm) and thus readily agglomerate, aporous thick film having few cracks could not be manufactured, butaccording to the method of the present invention, a porous thick filmhaving few cracks can be manufactured even if such a raw material isused; 3) because the average particle diameter of the porous thick filmobtained using the manufacturing method of the present invention is notmore than 200 nm, a resistive oxygen sensor having an excellent responsespeed can be obtained.

1. A method of manufacturing a porous thick film as an oxygen partialpressure detecting part of a resistive oxygen sensor comprising taking afine particle powder of an oxide containing cerium oxide as a rawmaterial powder, preparing a paste containing the oxide, printing thepaste onto a substrate by screen printing, calcining and sintering, themethod comprising: a heat treatment step of carrying out heat treatmentto effect particle growth from the average particle diameter of the rawmaterial powder to a particle diameter less than the average particlediameter of the ultimately obtained thick film; a step of mixing theparticle growth-effected powder with a solvent; a step of dispersingagglomerated particles in the solvent; a step of removing a precipitate;a step of evaporating off the solvent; and a step of mixing theresulting oxide with an organic binder to obtain the paste.
 2. Themethod according to claim 1, wherein the average particle diameter ofthe porous thick film is not more than 200 nm.
 3. The method accordingto claim 1, wherein the average particle diameter of the particlegrowth-effected powder obtained through the heat treatment step is atleast 45 nm.
 4. The method according to claim 1, wherein the averageparticle diameter of the raw material powder before the heat treatmentstep is at least 10 nm but less than 45 nm.
 5. The method according toclaim 1, wherein the raw material powder is subjected to heat treatmentat 880° C. to 920° C. in the heat treatment step.
 6. The methodaccording to claim 1, wherein the proportion by weight of the oxide inthe paste is adjusted to 10 to 30 wt %.
 7. The method according to claim1, wherein the fine particle powder of an oxide containing cerium oxideis a fine particle powder of an oxide containing cerium oxide andzirconium oxide.
 8. A cerium oxide-based porous thick film as an oxygenpartial pressure detecting part of a resistive oxygen sensor, the porousthick film manufactured using the method according to any of claims 1through 7, whereby the porous thick film has few cracks, has an averageparticle diameter of not more than 200 nm, and has an electricalconductivity of at least 10⁻³ S/m at 800° C.